1. Field of the Invention
The present invention relates to a free radical radiation curable inkjet ink suitable for digital food packaging printing.
2. Description of the Related Art
Short run packaging printing is shifting from conventional printing techniques, such as offset printing, to digital printing, where inkjet is one of the preferred technologies. In inkjet printing, tiny drops of ink are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a print head for ejecting the drops image-wise on an ink-receiver. Within ink jet for digital packaging printing, there is a clear evolution towards higher image quality and higher printing speeds. In order to satisfy these demands, a new print head design is required. These print heads require a specific ink design as they only can operate with very low viscous inks. The inks for high resolution, high speed short run digital packaging printing have to combine low viscosity, high curing speed and low migrating properties after curing.
Polymeric photoinitiators are known to improve the low migrating properties after curing, but also to increase the viscosity. Even polymeric photoinitiators with a compact design, like the hyperbranched photoinitiators disclosed by EP 1616921 A (AGFA), generally increase the viscosity of radiation curable compositions too much for the new print head designs.
Polymerizable photoinitiators are also known to improve the low migrating properties after curing. For example, EP 2161264 A (AGFA) discloses polymerizable Norrish Type II photoinitiators having a benzophenone group or a thioxanthone group in inkjet inks that exhibit low extractable amounts of the photoinitiators and their residues after curing.
In order to meet the demands for very low viscosity the amount of multifunctional monomers and oligomers have to be minimized. As a result, it becomes increasingly more difficult to avoid small amounts of residual unreacted monomers. These residual monomers are capable of migrating through especially low barrier substrates, typically polyolefines in general and polyethylene and biaxial oriented polypropylene more specifically or cause set off to the back side of substrates when prints are stored on roll.
Particularly interesting monomers for obtaining low viscous ink jet inks have been described in EP 0997508 A (AGFA) disclosing radiation curable ink compositions comprising radiation curable monomers containing vinylether and (meth)acrylate functions.
Radiation curable monomers containing vinylether and (meth)acrylate functions have further been disclosed as preferred diluents for food compliant radiation curable ink jet inks in WO 2010/069758 A (AGFA), WO 2010/133381 A (AGFA), and WO 2009/147057 A (AGFA).
US 2008045618 (HEXION) discloses a low viscosity hybrid curable ink comprising a vinyl ether component, a mono-acrylate component, a photocation polymerization initiator, and a free-radical photoinitiator, wherein the low viscosity ink formulation is free of epoxy functional monomer.
The rate and the completeness of a polymerization can be influenced through the type and concentration of monofunctional and polyfunctional monomers in an ink.
Monofunctional monomers have only one polymerizable functional group for taking part in the polymerization process and usually also exhibit a lower viscosity, whereby the polymerization can continue for a longer time than polyfunctional monomers, but at the end results in a certain amount of unreacted monomers trapped in the polymerized layer. Generally, polyfunctional monomers have larger probability of taking part in the polymerization because they have two or more polymerizable functional groups. However, because they can react more rapidly and frequently, vitrification of the layer occurs much faster leading to unreacted polyfunctional monomers getting trapped in the polymerized network. These trapped monomers contribute significantly to the extractables which limit the possibilities for inkjet printing food packaging applications.
WO 2009/053348 A (AGFA) discloses monomers for low viscous radiation curable ink jet inks for food packaging, comprising at least one acrylate and at least one free radical polymerizable group selected from the group consisting of a vinyl ether, an allyl ether and an allyl ester. By using polyfunctional monomers having polymerizable groups of a different reactivity, the vitrification can be controlled in a better manner during polymerization. Such monomers are sometimes referred to by the skilled person as vitrification control monomers.
However, it has been found that these vitrification control monomers as disclosed by WO 2009/053348 A (AGFA) are particularly prone to migrate through polyolefine substrates such as polyethylene or biaxial oriented polypropylene or even very thin PET foils, especially in low viscous compositions.
WO 2006/083344 (HENKEL) discloses viscous radiation curable compositions comprising (a) at least one (meth)acrylate; (b) at least one thiol component; (c) at least one organic acid present in a stabilizing amount; and (d) at least one curing initiator. The high viscosity can be directly deduced from paragraph [0044] since the diluent desirably has already a viscosity of about 50-500 mPa·s.
US 2009/000508 A (HEXION) discloses radiation curable inkjet inks comprising a radiation curable composition comprising about 0.1 to about 15 wt. % of an ethylenically unsaturated monofunctional monomer, about 30 to about 80 wt. % of an ethylenically unsaturated difunctional monomer, 0 to about 15 wt. % of an ethylenically unsaturated polyfunctional monomer, and 0 to about 15 wt. % of an ethylenically unsaturated urethane oligomer; a pigment dispersion; and a polymerization initiator; wherein the ink when containing ethylenically unsaturated urethane oligomer has an activation energy of fluidization of about 7 to 26 kJ/mol at a shear rate of about 1 to about 100,000 sec−1 and a temperature of about 25 to about 50° C.
EP1505090 A (NIPPON SHOKUBAI) discloses an activated energy ray-curable ink composition for ink-jet printing use, which contains a reactive diluent containing a vinyl ether group-containing (meth)acrylic ester and a hydroxyl group-containing polymerizable compound.